Speed up flushing the bucket cache on disk

The implementation of _gdbm_cache_flush becomes prohibitively
inefficient during extensive updates of large databases.  The
bug was reported at https://github.com/Perl/perl5/issues/19306.

To fix it, make sure that all changed cache entries are placed at
the head of the cache_mru list, forming a contiguous sequence.
This way a potentially long iteration over all cache entries can be
cut off at the first entry with ca_changed == FALSE.

This commit also gets rid of several superfluous fields in
struct gdbm_file_info:

- cache_entry

    Not needed, because the most recently used cache entry
    (cache_mru) is always the current one.

- bucket_changed

    dbf->cache_mru->ca_changed reflects the status of the current
    bucket.

- second_changed

    Not needed because _gdbm_cache_flush, which flushes all changed
    buckets, is now invoked unconditionally by _gdbm_end_update (and
    also whenever dbf->cache_mru changes).

* src/gdbmdefs.h (struct gdbm_file_info): Remove cache_entry.  The
current cache entry is cache_mru.
Remove bucket_changed, and second_changed.
All uses changed.
* src/proto.h (_gdbm_current_bucket_changed): New inline function.
* src/bucket.c (_gdbm_cache_flush): Assume all changed elements form
a contiguous sequence beginning with dbf->cache_mru.
(set_cache_entry): Remove.  All callers changed.
(lru_link_elem,lru_unlink_elem): Update dbf->bucket as necessary.
(cache_lookup): If the obtained bucket is not changed and is going
to become current, flush all changed cache elements.

* src/update.c (_gdbm_end_update): Call _gdbm_cache_flush unconditionally.
* src/findkey.c: Use dbf->cache_mru instead of the removed dbf->cache_entry.
* src/gdbmseq.c: Likewise.
* tools/gdbmshell.c (_gdbm_print_bucket_cache): Likewise.

* src/falloc.c: Use _gdbm_current_bucket_changed to mark the current
bucket as changed.
* src/gdbmstore.c: Likewise.
* src/gdbmdelete.c: Likewise.  Use _gdbm_current_bucket_changed.

* tests/gtcacheopt.c: Fix typo.
* tests/gtload.c: New option: -cachesize

1 files changed, 4 insertions, 4 deletions

diff --git a/src/gdbmdelete.c b/src/gdbmdelete.c
index 948c8a1..9206658 100644
--- a/src/gdbmdelete.c
+++ b/src/gdbmdelete.c
@@ -85,12 +85,12 @@ gdbm_delete (GDBM_FILE dbf, datum key)
     return -1;
 
   /* Set the flags. */
-  dbf->bucket_changed = TRUE;
+  _gdbm_current_bucket_changed (dbf);
 
   /* Invalidate data cache for the current bucket. */
-  dbf->cache_entry->ca_data.hash_val = -1;
-  dbf->cache_entry->ca_data.key_size = 0;
-  dbf->cache_entry->ca_data.elem_loc = -1;
+  dbf->cache_mru->ca_data.hash_val = -1;
+  dbf->cache_mru->ca_data.key_size = 0;
+  dbf->cache_mru->ca_data.elem_loc = -1;
 
   /* Do the writes. */
   return _gdbm_end_update (dbf);